This invention relates to semi-permeable gas separation membranes derived from oligomers containing reactive end groups and a process for separating gases using said membrane.
The use of membrances to separate gases is known in the art. Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and/or light hydrocarbons. Applications of particular interest include the separation of hydrogen or helium from gas mixtures such as mixtures containing nitrogen, carbon monoxide, carbon dioxide, water vapor, and/or light hydrocarbons. For example, the separation and recovery of hydrogen for recycle is often necessary in various hydrocracker, hydrotreater, and catalytic cracking processes used in the oil refinery industry. Other applications of interest include the separation of carbon dioxide from light hydrocarbons or other crude oil components as part of the tertiary oil recovery process. Additional applications include the recovery of an enriched oxygen stream from air for use in enhanced combustion or fermentation processes. Alternatively, an enriched nitrogen stream may be obtained from air for use as an inert atmosphere over flammable fluids or for food storage. Membranes can be used to achieve such separations.
Such membrane separation are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one gaseous component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other gaseous component of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating gaseous component or components and a stream which is depleted in the selectively permeating gaseous component or components. A relatively non-permeating gaseous component passes more slowly through the membrane than at least one other gaseous component of the mixture. An appropriate membrane material is chosen so that some degree of separation of the gas mixture can be achieved.
Membranes for gas separation have been fabricated from a wide variety of polymers, including certain cellulose esters and ethers, aromatic polyimides, polyaramides, plyarylates, polysulfones, polyethersulfones, polyesters, and polycarbonates. An ideal gas separation membrane is characterized by the ability to operate under high temperatures and/or pressures with minimal compaction and creep, while processing a high gas separation factor, that is, gas selectivity, and high gas permeability. Solvent resistance is also preferred; however, gas separation membranes also are preferably fabricated from polymers which are easily processed.
The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low gas separation factors. In the case, a choice between a high gas separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some of the polymers previously used for membranes suffer from the disadvantage of poor performance under high operating temperatures and pressures due to compaction and creep experienced under such operating conditions over time. However, those polymers capable of operating at high temperatures and pressures for prolonged periods are typically very difficult to fabricate into membranes due to low solubility in common solvents and high melt and solution viscosities. Some polymers, particularly those containing substituent groups favorable for gas separation such as certain halo-substituents, may be very difficult to synthesize with sufficiently high molecular weight to be useful materials for membranes. Such polymers also tend to be expensive.
A membrane capable of separating gas mixtures which possesses good gas selectivity and gas permeability, ability to operate under extreme conditions of temperature and pressure with minimal compaction and creep, improved solvent resistance, and ease of fabrication is needed.